Preparation method for olefin epoxidation catalyst and applications thereof

ABSTRACT

Disclosed in the present invention are a preparation method for an olefin epoxidation catalyst and applications thereof. The method comprises: loading an auxiliary metal salt onto a silica gel carrier, and carrying out a drying treatment to the silica gel carrier; loading a titanium salt (preferably TiCl4) onto the silica gel carrier by a chemical vapor deposition method; calcining to obtain a silica gel on which the auxiliary metal oxide and Ti species are loaded; obtaining an catalyst precursor (Ti-MeO—SiO2 composite oxide) by water vapor washing; loading alkyl silicate (preferably tetraethyl orthosilicate) onto the surface of the catalyst precursor by a chemical vapor deposition method and calcining the catalyst precursor to obtain a Ti-MeO—SiO2 composite oxide with the surface coated with a SiO2 layer; and carrying out a silylanization treatment to obtain the catalyst. The catalyst can be applied to a chemical process of propylene epoxidation to prepare propylene oxide, and has an average selectivity to PO up to 96.7%, the method of the present invention and the applications thereof have industrial application prospects.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a national stage filing under 35 U.S.C. § 371 ofInternational Patent Application No.: PCT/CN2018/088175, filed May 24,2018, which claims priority to Chinese Patent Application No.201710379007.2, filed May 25, 2017, the entire contents of all of whichare hereby incorporated by reference herein.

TECHNICAL FIELD

The present disclosure relates to a preparation method for an olefinepoxidation catalyst, specifically a preparation method for aSiO₂—Ti-MeO—SiO₂ composite oxide catalyst and a catalyst preparedaccording to the method, and further relates to use of the catalyst incatalyzing propylene epoxidation to prepare propylene oxide.

BACKGROUND

Co-oxidation methods, also known as Halcon methods, are importantmethods for producing propylene oxide, comprising isobutane co-oxidationmethod (PO/TBA) and ethylbenzene co-oxidation method (PO/SM). First ofall, peroxide is obtained by oxidizing isobutane or ethylbenzenerespectively, then propylene oxide is obtained by oxidizing propylene,and tert-butyl alcohol or styrene is co-produced.

Co-oxidation methods overcome the disadvantages such as strongcorrosiveness, large production of polluted water of the chlorohydrinmethod, and have advantages such as low product cost (co-products sharethe cost) and less environmental pollution. Co-oxidation methods havebeen developed rapidly around the world since the methods wereindustrialized in 1969. At present, the capacity of producing propyleneoxide by a co-oxidation method accounts for about 55% of the totalcapacity in the world. In the co-oxidation methods, PO/SM processco-produces styrene, which is a chemical product produced in bulk and isan important monomer for synthesizing resin and rubber. Due to therelatively broad market of the co-product, this process is spreadingfaster and faster.

The PO/SM processes can be divided into homogeneous PO/SM processes andheterogeneous PO/SM processes according to the different catalysts usedin the key procedures (epoxidation procedures) of the processes. Thecatalysts used in the epoxidation procedures of the heterogeneous PO/SMprocesses are Ti—SiO₂ composite oxide catalysts. The main steps of thepreparation methods disclosed in U.S. Pat. No. 3,829,392 andUS2003166951A1 as well as Chinese Patent Applications CN1894030A andCN1720100A are as follows: a silica gel carrier was dried first, andthen using N₂ or other inert gases to introduce titanium halide vaporinto a reaction tube to react with silica gel (this step is called achemical vapor deposition), high temperature calcination, and finally acatalyst was prepared through steps such as water washing. The catalystprepared by this method has problems such as the Ti active center iseasily lost during use, the catalyst activity decreases rapidly and thecatalyst has a short life.

In order to solve the above problems, it is necessary to seek a newpreparation method for propylene epoxidation catalyst to reduce the lossof Ti active center and improve the service life of the catalyst.

SUMMARY OF THE DISCLOSURE

The object of the present disclosure is to provide a preparation methodfor an olefin epoxidation catalyst. The olefin epoxidation catalystprepared by the preparation method of the present disclosure can reducethe loss of Ti active center during use and improve the service life ofthe catalyst.

In the present disclosure, the catalyst is obtained as follows: anauxiliary metal salt is loaded onto a silica gel carrier first, dried,and then a chemical vapor deposition is carried out, the thus obtainedproduct is calcined to load Ti species, washed with water, then achemical vapor deposition and calcination are carried out again to loada SiO₂ layer onto the surface of Ti-MeO—SiO₂, and finally asilylanization treatment is carried out. The catalyst prepared by thepreparation method of the present disclosure can reduce the loss of theTi active center and improve the service life of the catalyst duringuse, especially when applied for catalyzing propylene epoxidation toproduce propylene oxide.

Another object of the present disclosure is to provide use of thecatalyst (also referred to as a SiO₂—Ti-MeO—SiO₂ composite oxidecatalyst) prepared by said method, the catalysts can be used as acatalyst for olefin epoxidation to produce an epoxide, especially acatalyst for propylene epoxidation to produce propylene oxide, and thecatalyst not only has good catalyst activity, the catalyst also has highaverage selectivity to PO (propylene oxide).

In order to achieve the above objects, the present disclosure adopts thefollowing technical solutions:

A preparation method for an olefin epoxidation catalyst, which comprisesthe following steps:

(1) loading an auxiliary metal salt onto a silica gel carrier to obtaina silica gel carrier A modified by the auxiliary metal salt; in somespecific embodiments, the auxiliary metal salt can be loaded onto thesilica gel carrier using a impregnation method, such as anincipient-wetness impregnation method;

(2) carrying out a drying treatment for the A obtained in step (1);

(3) carrying out a chemical vapor deposition for the dried A using atitanium salt vapor, preferably TiCl₄ vapor to obtain a silica gel B onwhich the auxiliary metal salt and the titanium salt, preferably TiCl₄are loaded; in some preferred embodiments, in step (3), the dried A ischarged into a reaction tube (preferably a fixed bed reactor), andpreferably nitrogen is used to introduce the titanium salt vapor intothe reaction tube to carry out the chemical vapor deposition;

(4) calcining the B obtained in step (3) to obtain a silica gel C onwhich the auxiliary metal salt and Ti species are loaded; preferably,the calcination is carried out in N₂ atmosphere;

(5) carrying out a water vapor washing for the C obtained in step (4),the product obtained by washing is referred to as Ti-MeO—SiO₂ compositeoxide;

(6) carrying out another vapor deposition for the Ti-MeO—SiO₂ compositeoxide obtained in step (5) using an alkyl silicate vapor, preferablytetraethyl orthosilicate vapor to obtain the Ti-MeO—SiO₂ composite oxideD having a silicon-containing compound loaded on the surface of thecomposite oxide; preferably, the vapor deposition is carried out in areaction tube, and the alkyl silicate vapor enters the reaction tube tocarry out the vapor deposition for the Ti-MeO—SiO₂ composite oxide;further preferably, the alkyl silicate vapor is introduced into thereaction tube using nitrogen to carry out the vapor deposition;

(7) calcining the D obtained in step (6) to obtain a Ti-MeO—SiO₂composite oxide having a SiO₂ layer coated on the surface of thecomposite oxide, this product is referred to as SiO₂—Ti-MeO—SiO₂;

(8) carrying out a silylanization treatment for the SiO₂—Ti-MeO—SiO₂obtained in step (7).

The product codes “A”, “B”, “C”, “D” appeared in the preparation methodof the present disclosure do not have special meanings and are used asproduct codes only for ease of description. The Ti species describedherein are terms commonly used in the art, the Ti species comprise tetracoordinate skeletal titanium, free titanium dioxide.

Further, in some specific embodiments, steps (3)-(8) are all carried outin the same reaction tube.

The auxiliary metal salt in step (1) of the present disclosure can beone or more of Ce(NO₃)₃, Pr(NO₃)₃, Tb(NO₃)₃ and La(NO₃)₃. Using theamount of the metal oxide of the auxiliary metal salt for calculation,the auxiliary metal salt is added in an amount preferably ranging from0.6-2.4 wt % based on the mass of the silica gel carrier. Said metaloxide refers to an oxide obtained from the decomposition afterhigh-temperature calcination of the auxiliary metal salt, for example,the oxides corresponding to Ce(NO₃)₃, Pr(NO₃)₃, Tb(NO₃)₃ and La(NO₃)₃are CeO₂, Pr₆O₁₁, TbO₂ and La₂O₃ respectively.

The impregnation method in step (1) can be an incipient-wetnessimpregnation method or other impregnation methods, and is notparticularly limited as long as the method can load the auxiliary metalsalt on the silica gel carrier. The specific operations of loading theauxiliary metal salt using an incipient-wetness impregnation method, forexample, can be carried out as follows: first of all, measuring thewater absorption per unit mass of the silica gel carrier, calculatingthe saturated water absorption of the silica gel carrier, and recordingthe saturated water absorption as a; weighing a certain amount ofauxiliary metal salt and dissolving the salt in a grams of water, andrecording the water solution as b solution; then evenly spraying bsolution on the surface of the silica gel carrier, then allowing tosettle, drying specifically, such as allowing to settle for 2-5 hoursthen drying at 80-120° C.; specific impregnation operations areconventional means in the art, and will not be described in detail.

An auxiliary metal salt is added in step (1) of the present disclosure,and the auxiliary metal salt is preferably one or more of Ce(NO₃)₃,Pr(NO₃)₃, Tb(NO₃)₃ and La(NO₃)₃. The purpose of adding an auxiliarymetal salt is to utilize the alkalinity of the oxide formed from theauxiliary metal salt during the subsequent calcination process, therebyneutralizing the acidity of the free TiO₂ which is inevitably producedin step (4) and the subsequent steps, reducing self-decompositionreactions of peroxides and improving catalyst selectivity.

The silica gel carrier used in step (1) of the present disclosure is aC-type silica gel, and the specific shape thereof can be but is notlimited to a spherical shape or a block etc., and there's no specialrequirement for the specific shape, for example, the silica gel can bebut is not limited to an irregular blocky C-type silica gel. Preferably,the silica gel carrier used in step (1) has a specific surface area of100-350 m²/g, an average pore diameter of 8-11 nm, a pore volume of0.7-1.2 ml/g, a Na₂O impurity content of <100 ppm, a Fe₂O₃ impuritycontent of <500 ppm and a size of a spherical equivalent diameter of0.5-2 mm.

The main purpose of drying in step (2) of the present disclosure is toremove free water in the silica gel carrier, preferred dryingtemperature is 150-240° C. and preferred drying time is 120 min-240 min.Preferably, in step (2), the auxiliary metal salt modified silica gelcarrier A obtained in step (1) is dried using a drying gas, and thedrying gas used for drying can be any gas that does not react withsilica gel, such as air or nitrogen; the drying in step (2) can becarried out specifically in a reaction tube, and the flow rate of thedrying gas in the reaction tube is preferably 1.0-3.0 cm/s.

The step (3) of the present disclosure is a chemical vapor deposition,specifically, the chemical vapor deposition of this step can be carriedout in a reaction tube. The titanium salt vapor, preferably TiCl₄ vaporin the vaporization tank is introduced into the reaction tube using aninert gas, preferably N₂. The TiCl₄ vapor interacts with the associatedhydroxy on the surface of the silica gel to carry out the followingreaction:

˜O—Si—OH+TiCl₄→˜O—Si—O—Ti—Cl

wherein Ti is loaded on the silica gel carrier in an amount ranges from0.1-5.0 wt %, preferably 2.5-4.5 wt % based on the weight of the silicagel carrier (i.e., the amount of loaded Ti is calculated based on theweight of the silica gel carrier used in step (1)), the Ti describedherein is based on the Ti element in the used titanium salt vapor. Insome specific embodiments, the titanium salt is contained in avaporization tank to provide titanium salt vapor, and the titanium saltvapor, preferably TiCl₄ vapor has a temperature of 137° C.-150° C., i.e.the titanium salt vapor is preheated to this temperature; the flow rateof the inert gas, preferably N₂ in the reaction tube is 0.05-2.0 cm/s,preferably 0.50-1.35 cm/s, the temperature of the reaction between thetitanium salt vapor, preferably TiCl₄ vapor and the associated hydroxyon the surface of the silica gel is 150-300° C., and the deposition timeis 120-240 min.

In some preferred embodiments, the calcination carried out in N₂atmosphere in step (4) of the present disclosure is carried out at acalcination temperature ranges from 450-700° C., preferably 500-600° C.,the temperature is increased in a rate ranges from 1.5-3° C./min, andthe calcination time is 30-240 min, preferably 120-180 min, the flowrate of N₂ is 0.05-2.0 cm/s, preferably 1-2.5 cm/s. The main purpose ofcalcination is to introduce the Ti species adsorbed on the surface ofthe silica gel into the silica gel skeleton to form a tetra-coordinatedskeletal Ti (or a Ti═O tetrahedron) active center, to stabilize theactive center, and some of the Cl elements form HCl gas duringcalcination process and are removed from the surface of the silica gelcarrier.

The step (5) of the present disclosure is water washing, and the purposethereof is to remove the Cl element adsorbed on the surface of thesilica gel carrier using water vapor, reducing or eliminating theinfluence of Cl element on the performance of the catalyst; the waterwashing process can be carried out specifically in the reaction tube.Preferably, a certain amount of water vapor is introduced into thereaction tube using an inert gas, preferably N₂, and the Cl element onthe surface of the silica gel carrier is interacted with the water vaporto form HCl gas to be removed from the surface of the silica gelcarrier. In some specific embodiments, the water vapor comes from avaporization tank, which has a temperature of 100-200° C. (i.e., thewater vapor is preheated to this temperature), preferably 120-180° C.;the water washing time is 180-240 min, and the flow rate of the inertgas, preferably N₂ is 1-2.5 cm/s. The molar ratio of water vapor to Tiis 20-150:1, preferably 50-100:1, and the amount of Ti in this molarratio is based on the amount of the Ti element in the titanium saltvapor used in step (3); water washing with a preferred molar ratio ofwater vapor can remove Cl effectively and completely.

The step (6) of the present disclosure is also a chemical vapordeposition, and the main purpose thereof is to load an alkyl silicate,preferably tetraethyl orthosilicate onto the surface of the silica gelcarrier. Preferably, the chemical vapor deposition of this step iscarried out in a reaction tube, and an alkyl silicate vapor, preferablytetraethyl orthosilicate vapor is introduced into the reaction tubeusing an inert gas, preferably N₂; in some specific embodiments, thealkyl silicate vapor, preferably tetraethyl orthosilicate vapor comesfrom a vaporization tank that is heated to a temperature of 166-200° C.,the flow rate of the inert gas, preferably N₂ in the reaction tube is0.05-2.0 cm/s, preferably 0.5-1.0 cm/s, the reaction temperature is166-200° C., the reaction time is 120-180 min, and the weight ratio ofalkyl silicate, preferably tetraethyl orthosilicate to silica gelcarrier is 0.5-1:1, the weight of the silica gel carrier in this weightratio is based on the weight of the silica gel carrier used in step (1).

The purpose of the step (7) of the present disclosure is to calcine andremove the ligand alkyl ester in the alkyl silicate in air atmosphere,the ligand alkyl ester forms an oxycarbide and is then removed, a SiO₂film (or a SiO₂ layer) is further formed on the surface of the silicagel carrier, and a SiO₂—Ti-MeO—SiO₂ sandwich structure is formed, whichcan protect Ti species, reducing (decreasing) the loss (loss rates) ofTi active centers, and improving the stability of the catalyst. In somepreferred embodiments, in step (7), the calcination is carried out witha heating rate of 1.5-3° C./min, a calcination temperature of 500-700°C., a calcination time of 30-120 min, and the flow rate of air is 0.5-1cm/s.

The step (8) of the present disclosure is a silylanization treatment,and the silylanization treatment can be specifically carried out in areaction tube, a silylanization reagent vapor is introduced into thereaction tube using an inert gas, preferably N₂, and the chemicalreaction is carried out as follows:

˜O—Si—OH+Si(CH₃)₃—NH—Si(CH₃)₃→˜O—Si—O—Si(CH₃)₃

The purpose of the silylanization treatment is to increase thehydrophobicity of the catalyst surface, reducing the decompositionability of the catalyst to peroxide, and improving the selectivity ofthe catalyst; the silylanization reagent is preferably hexamethyldisilylamine, hexamethyl disilylamine which is used in an amount of 5 wt%-15 wt % based on the weight of the silica gel carrier (i.e., based onthe weight of the silica gel carrier used in step (1)); preferably, thetemperature of hexamethyl disilylamine is 126-150° C., thesilylanization temperature is 200-300° C., the flow rate of the inertgas, preferably N₂ in the reaction tube is 0.5-1 cm/s, and thesilylanization time is 60-180 min.

The catalyst prepared by the preparation method of the presentdisclosure is used for catalyzing propylene epoxidation to preparepropylene oxide. The preferred process conditions are as follows: thereaction temperature is 40-120° C., the pressure is 2-4.5 MPa (gaugepressure), the molar ratio of propylene to ethylbenzene hydroperoxide(EBHP) is 3-10:1, and the mass space velocity is 1-5 h⁻¹.

The present disclosure has the following advantageous effects:

(1) A layer of SiO₂ is further deposited on the surface of Ti-MeO—SiO₂composite oxide by chemical vapor deposition, a layer of protective SiO₂shell is formed, and a SiO₂—Ti-MeO—SiO₂ sandwich structure is formed,which can protect Ti species, reducing (decreasing) the loss (lossrates) of Ti active centers, and improving the stability of thecatalyst; (2) by modifying the silica gel carrier though adding a metalauxiliary thereto, the metal auxiliary synergizes with Ti active centersto improve the activity and the selectivity to propylene oxide of thecatalyst, the average selectivity of the catalyst to PO is up to 96.7%,which reduces the consumption of only propylene of PO products; (3) Theprocess flow of the present disclosure is simple to control, easy toindustrialize, and has great prospects for industrialization.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram showing the process flow of a catalystpreparation device.

Description of the reference signs: 1 represents a alkyl silicatevaporization tank, 2 represents a TiCl₄ vaporization tank, 3 representsa water and silylanization reagent vaporization tank; 4 represents anexhaust gas absorption tank; and 5 represents a reaction tube.

DETAILED DESCRIPTION

In order to understand the present disclosure better, the presentdisclosure will be further illustrated below with reference to theembodiments, but the content of the present disclosure is not limitedthereto.

The Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES)used in the examples of the present disclosure was produced by AgilentTechnologies, model 720 ICP-OES;

In the examples of the present disclosure, the content of PO (propyleneoxide) in the reaction liquid and the exhaust gas absorption liquid wasanalyzed by gas chromatography, and the conversion rate of EBHP(ethylbenzene hydroperoxide) was analyzed by iodimetry. The conditionsof chromatography are shown in Table 1.

TABLE 1 Operating conditions of Chromatography Chromatographic columnAgilent 19091N-133 (30 m × 250 μm × 0.25 μm) Flow rate of H₂ 35 mL/minFlow rate of air 350 mL/min Flow rate of makeup gas (N₂) 25 mL/minHeater 270° C. Column box 250° C. Heating procedure Initial temperature:50° C. Heating program: 50-100° C., 15° C./min, maintaining for 0 min,100-250° C., 20° C./min, maintaining for 2 min Split ratio of injectionport 30:1 Temperature of FID detector 270° C.

The content of PO was determined by internal standard method. The liquidphase concentration was determined using DMF as the solvent and DT(dioxane) as the internal standard substance. The internal standardcurve of PO and DT was determined to be y=0.6985x-0.0046, R²=0.999; theconcentration of PO in gas phase absorption liquid was determined usingtoluene as the internal standard substance, the internal standard curveof PO and toluene was determined to be y=2.161x+0.0002, R²=0.999.

Concentration of PO in liquidphase=(0.6985×(A_(PO)/A_(DT))−0.0046)×0.01×dilution ratio, A representsthe peak area, the same below;

Content of PO in liquid phase=concentration of PO in liquid phase×massof sample in liquid phase;

Concentration of PO in gasphase=(2.162×(A_(PO)/A_(toluene))+0.0002)×mass of toluene;

Content of PO in gas phase=concentration of PO in gas phase×total amountof absorption liquid/amount of sample in gas phase;

Total production amount of PO=content of PO in gas phase+content of POin liquid phase;

Selectivity to PO=total production amount of PO/amount of POtheoretically produced from propylene oxidized by EBHP (ethylbenzenehydroperoxide)×100%.

The conversion rate of EBHP was titrated by iodimetry and measured by atitrator.

Conversion rate of EBHP=(initial value of EBHP−residual amount ofEBHP)/initial value of EBHP.

Residual amount of EBHP=(titration end point−blank)×C_(Na) ₂ _(S) ₂ _(O)₃ ×0.001×0.5×142×total amount of liquid sample/sample amount fortitration, wherein C_(Na) ₂ _(S) ₂ _(O) ₃ is the concentration of sodiumthiosulfate.

The silica gel carrier used in the examples or the comparative examplesis an irregular blocky C-type silica gel carrier, which is provided byQingdao GuiChuang Fine Chemical Co., Ltd., and has a silica gel particlesize (spherical equivalent diameter) of 0.6-1.1 mm, a specific surfacearea of 256 m²/g, an average pore diameter of 9.2 nm, a pore volume of0.94 ml/g, a water absorption rate of 1.12 g water/g of silica gel(saturated water absorption), a Na₂O impurity content of 89 ppm and aFe₂O₃ impurity content of 327 ppm.

The process conditions for propylene epoxidation to produce propyleneoxide in the examples and comparative examples are as follows: theoxidant is ethylbenzene hydroperoxide (EBHP), the reaction tube is afixed bed reactor with an inner diameter of 24 mm, and the catalyst isloaded in an amount of 10 g; the molar ratio of propylene to EBHP is5:1, the mass space velocity is 4 h⁻¹; the initial reaction temperatureis 50° C., and the reaction temperature is gradually increased accordingto the conversion rate of EBHP (the conversion rate of EBHP wasguaranteed to be >99%).

Example 1

0.36 g of Ce(NO₃)₃ was weighed and dissolved in 33.6 g of distilledwater, sprayed onto 30 g of silica gel carrier, to settle for 2 hoursand then dried at 80° C. The product obtained in this step was calledauxiliary metal salt modified silica gel carrier A (or referred to asproduct A or A for short).

As shown in FIG. 1, the silica gel carrier on which Ce(NO₃)₃ was loaded(i.e., product A) was charged into reaction tube 5 and dried at 180° C.for 120 min in N₂ atmosphere, and the linear velocity of N₂ in reactiontube 5 was 1.2 cm/s. Dried A was obtained in this step.

Chemical vapor deposition of TiCl₄: 2.98 g of TiCl₄ was added to TiCl₄vaporization tank 2 which was then heated at a temperature of 137° C.,TiCl₄ vapor was introduced into reaction tube 5 using N₂ to react withthe silica gel, the linear velocity of N₂ in reaction tube 5 was 0.5cm/s, the deposition time was 120 min (the reaction temperature of thisstep was the same as the drying temperature in the previous step, whichwas 180° C.); the product obtained in this step was called auxiliarymetal salt and TiCl₄ loaded silica gel B (referred to as silica gel B orB for short).

Calcination: the temperature was raised to 500° C. at a heating rate of2° C./min, the linear velocity of N₂ in reaction tube 5 was 1 cm/s, andthe silica gel B was calcined for 120 min; the product obtained in thisstep was called auxiliary metal oxide and Ti species loaded silica gel C(or referred to as C for short).

Water washing: 14.1 g of distilled water was added to water andsilylanization reagent vaporization tank 3 which was then heated at atemperature of 120° C., water vapor was introduced into reaction tube 5using N₂ for water washing the obtained silica gel C, the linearvelocity of N₂ in the reaction tube was 1 cm/s and the water washingtime was 180 min; the product obtained by water vapor washing wasreferred to as a Ti-MeO—SiO₂ composite oxide.

Chemical vapor deposition of alkyl silicate: 15 g of tetraethylorthosilicate was added to alkyl silicate vaporization tank 1 which wasthen heated at a temperature of 166° C., tetraethyl orthosilicate vaporwas introduced into the reaction tube using N₂ to react with the silicagel, the linear velocity of N₂ in the reaction tube was 0.5 cm/s, andthe deposition time was 120 min (the reaction temperature of this stepwas the same as the temperature at which the alkyl silicate vaporizationtank was heated, which was 166° C.); the product obtained in this stepwas called a Ti-MeO—SiO₂ composite oxide D having a silicon-containingcompound loaded on the surface of the composite oxide (or referred to asproduct D);

Calcination: the temperature was raised to 500° C. at a heating rate of3° C./min, the linear velocity of air in the reaction tube was 0.5 cm/s,and the product D was calcined for 120 min; a product having a SiO₂layer coated on the surface was obtained by calcination, the product wascalled a Ti-MeO—SiO₂ composite oxide having a SiO₂ layer coated on thesurface, or is referred to as SiO₂—Ti-MeO—SiO₂.

Silylanization treatment: 4.5 g of hexamethyl disilylamine was added towater and silylanization reagent vaporization tank 3 which was thenheated at a temperature of 130° C., the hexamethyl disilylamine vaporwas introduced into the reaction tube using N₂ to react with the silicagel (the silica gel refers to the product obtained by calcination,SiO₂—Ti-MeO—SiO₂), the linear velocity of N₂ in the reaction tube was 1cm/s, the silylanization temperature was 200° C., and the silylanizationtime was 180 min; the obtained catalyst is referred to as TS-C1.

Exhaust gas absorption tank 4 was used for absorbing TiCl₄ that was notloaded on the carrier and HCl gas generated during the calcination andwater washing procedure.

The TS-C1 was evaluated. The catalyst was used for propylene epoxidationto produce propylene oxide, operated continuously for 1000 hr, thereaction temperature was raised from the initial 50° C. to 80° C., andsampled for gas chromatography analysis. The EBHP conversion ratewas >99.9%, the highest selectivity to PO reached 96.4%, the averageselectivity reached 95.2%. The product was collected for ICP-OESanalysis and no catalyst component Ti was found.

Example 2

0.48 g of Pr(NO₃)₃ was weighed and dissolved in 33.6 g of distilledwater, sprayed onto 30 g of silica gel carrier, allowed to settle for 2hours and then dried at 80° C.

As shown in FIG. 1, the silica gel carrier on which Pr(NO₃)₃ was loadedwas charged into a reaction tube and dried at 200° C. for 180 min in N₂atmosphere, and the linear velocity of N₂ in the reaction tube was 1.6cm/s.

Chemical vapor deposition of TiCl₄: 3.57 g of TiCl₄ was added to a TiCl₄vaporization tank which was then heated at a temperature of 140° C.,TiCl₄ vapor was introduced into the reaction tube using N₂ to react withthe silica gel, the linear velocity of N₂ in the reaction tube was 0.7cm/s, the deposition time was 180 min (the reaction temperature of thisstep was the same as the drying temperature in the previous step, whichwas 200° C.);

Calcination: the temperature was raised to 550° C. at a heating rate of2° C./min, the linear velocity of N₂ in the reaction tube was 1.5 cm/s,the calcination lasted for 180 min;

Water washing: 26.7 g of distilled water was added to a water andsilylanization reagent vaporization tank which was then heated at atemperature of 140° C., water vapor was introduced into the reactiontube using N₂ for water washing, the linear velocity of N₂ in thereaction tube was 1.5 cm/s and the water washing time was 180 min;

Chemical vapor deposition of alkyl silicate: 21 g of tetraethylorthosilicate was added to a alkyl silicate vaporization tank which wasthen heated at a temperature of 170° C., tetraethyl orthosilicate vaporwas introduced into the reaction tube using N₂ to react with the silicagel, the linear velocity of N₂ in the reaction tube was 0.8 cm/s, andthe deposition time was 150 min (the reaction temperature of this stepwas the same as the temperature at which the alkyl silicate vaporizationtank was heated, which was 170° C.);

Calcination: the temperature was raised to 550° C. at a heating rate of3° C./min, the linear velocity of air in the reaction tube was 0.7 cm/s,the calcination lasted for 60 min;

Silylanization treatment: 3 g of hexamethyl disilylamine was added to awater and silylanization reagent vaporization tank which was then heatedat a temperature of 135° C., hexamethyl disilylamine vapor wasintroduced into the reaction tube using N₂ to react with the silica gel,the linear velocity of N₂ in the reaction tube was 0.7 cm/s, thesilylanization temperature was 200° C., and the silylanization time was150 min; the obtained catalyst was referred to as TS-C2.

The TS-C2 was evaluated. The catalyst was used for propylene epoxidationto produce propylene oxide, operated continuously for 1550 hr, thereaction temperature was raised from the initial 50° C. to 85° C., andsampled for gas chromatography analysis. The EBHP conversion ratewas >99.9%, the highest selectivity to PO reached 96.7%, the averageselectivity reached 95.1%. The product was collected for ICP-OESanalysis and no catalyst component Ti was found.

Example 3

0.6 g of Tb(NO₃)₃ was weighed and dissolved in 33.6 g of distilledwater, sprayed onto 30 g of silica gel carrier, allowed to settle for 2hours and then dried at 80° C.

As shown in FIG. 1, the silica gel carrier on which Tb(NO₃)₃ was loadedwas charged into a reaction tube and dried at 240° C. for 240 min in N₂atmosphere, and the linear velocity of N₂ in the reaction tube was 2cm/s.

Chemical vapor deposition of TiCl₄: 4.17 g of TiCl₄ was added to a TiCl₄vaporization tank which was then heated at a temperature of 145° C.,TiCl₄ vapor was introduced into the reaction tube using N₂ to react withthe silica gel, the linear velocity of N₂ in the reaction tube was 0.8cm/s, the deposition time was 200 min (the reaction temperature of thisstep was the same as the drying temperature in the previous step, whichwas 240° C.);

Calcination: the temperature was raised to 600° C. at a heating rate of2° C./min, the linear velocity of N₂ in the reaction tube was 2 cm/s,the calcination lasted for 200 min;

Water washing: 33.5 g of distilled water was added to a water andsilylanization reagent vaporization tank which was then heated at atemperature of 160° C., water vapor was introduced into the reactiontube using N₂ for water washing, the linear velocity of N₂ in thereaction tube was 2 cm/s and the water washing time was 200 min;

Chemical vapor deposition of alkyl silicate: 27 g of tetraethylorthosilicate was added to a alkyl silicate vaporization tank which wasthen heated at a temperature of 180° C., tetraethyl orthosilicate vaporwas introduced into the reaction tube using N₂ to react with the silicagel, the linear velocity of N₂ in the reaction tube was 0.9 cm/s, andthe deposition time was 180 min (the reaction temperature of this stepwas the same as the temperature at which the alkyl silicate vaporizationtank was heated, which was 180° C.);

Calcination: the temperature was raised to 600° C. at a heating rate of3° C./min, the linear velocity of air in the reaction tube was 0.8 cm/s,the calcination lasted for 60 min;

Silylanization treatment: 2.4 g of hexamethyl disilylamine was added toa water and silylanization reagent vaporization tank which was thenheated at a temperature of 140° C., hexamethyl disilylamine vapor wasintroduced into the reaction tube using N₂ to react with the silica gel,the linear velocity of N₂ in the reaction tube was 0.5 cm/s, thesilylanization temperature was 250° C., and the silylanization time was120 min; the obtained catalyst was referred to as TS-T3.

The TS-T3 was evaluated. The catalyst was used for propylene epoxidationto produce propylene oxide, operated continuously for 1800 hr, thereaction temperature was raised from the initial 50° C. to 90° C., andsampled for gas chromatography analysis. The EBHP conversion ratewas >99.9%, the highest selectivity to PO reached 97.8%, the averageselectivity reached 96.7%. The product was collected for ICP-OESanalysis and no catalyst component Ti was found.

Example 4

0.72 g of La(NO₃)₃ was weighed and dissolved in 33.6 g of distilledwater, sprayed onto 30 g of silica gel carrier, allowed to settle for 2hours and then dried at 80° C.

As shown in FIG. 1, the silica gel carrier on which La(NO₃)₃ was loadedwas charged into a reaction tube and dried at 220° C. for 120 min in N₂atmosphere, and the linear velocity of N₂ in the reaction tube was 2.5cm/s.

Chemical vapor deposition of TiCl₄: 5.36 g of TiCl₄ was added to a TiCl₄vaporization tank which was then heated at a temperature of 150° C.,TiCl₄ vapor was introduced into the reaction tube using N₂ to react withthe silica gel, the linear velocity of N₂ in the reaction tube was 1.35cm/s, the deposition time was 240 min (the reaction temperature of thisstep was the same as the drying temperature in the previous step, whichwas 220° C.);

Calcination: the temperature was raised to 550° C. at a heating rate of2° C./min, the linear velocity of N₂ in the reaction tube was 2.5 cm/s,and the calcination lasted for 240 min;

Water washing: 50.7 g of distilled water was added to a water andsilylanization reagent vaporization tank which was then heated at atemperature of 180° C., water vapor was introduced into the reactiontube using N₂ for water washing, the linear velocity of N₂ in thereaction tube was 2.5 cm/s and the water washing time was 240 min;

Chemical vapor deposition of alkyl silicate: 30 g of tetraethylorthosilicate was added to a alkyl silicate vaporization tank which wasthen heated at a temperature of 200° C., the tetraethyl orthosilicatevapor was introduced into the reaction tube using N₂ to react with thesilica gel, the linear velocity of N₂ in the reaction tube was 1 cm/s,and the deposition time was 180 min (the reaction temperature of thisstep was the same as the temperature at which the alkyl silicatevaporization tank was heated, which was 200° C.);

Calcination: the temperature was raised to 700° C. at a heating rate of3° C./min, the linear velocity of air in the reaction tube was 1 cm/s,the calcination lasted for 30 min;

Silylanization treatment: 1.8 g of hexamethyl disilylamine was added toa water and silylanization reagent vaporization tank which was thenheated at a temperature of 150° C., hexamethyl disilylamine vapor wasintroduced into the reaction tube using N₂ to react with the silica gel,the linear velocity of N₂ in the reaction tube was 0.6 cm/s, thesilylanization time was 150 min; the silylanization temperature was 300°C., the obtained catalyst was referred to as TS-L4.

The TS-L4 was evaluated. The catalyst was used for propylene epoxidationto produce propylene oxide, operated continuously for 1200 hr, thereaction temperature was raised from the initial 50° C. to 75° C., andsampled for gas chromatography analysis. The EBHP conversion ratewas >99.9%, the highest selectivity to PO reached 96.8%, the averageselectivity reached 95.6%. The product was collected for ICP-OESanalysis and no catalyst component Ti was found.

Comparative Example 1

0.6 g of Tb(NO₃)₃ was weighed and dissolved in 33.6 g of distilledwater, sprayed onto 30 g of silica gel carrier, allowed to settle for 2hours and then dried at 80° C.

The silica gel carrier on which Tb(NO₃)₃ was loaded was charged into thereaction tube and dried at 240° C. for 240 min in N₂ atmosphere, and thelinear velocity of N₂ in the reaction tube was 2 cm/s.

Chemical vapor deposition of TiCl₄: 4.17 g of TiCl₄ was added to a TiCl₄vaporization tank which was then heated at a temperature of 145° C.,TiCl₄ vapor was introduced into the reaction tube using N₂ to react withthe silica gel, the linear velocity of N₂ in the reaction tube was 0.8cm/s, the deposition time was 200 min (the reaction temperature of thisstep was the same as the drying temperature in the previous step, whichwas 240° C.);

Calcination: the temperature was raised to 600° C. at a heating rate of2° C./min, the linear velocity of N₂ in the reaction tube was 2 cm/s,and calcination lasted for 200 min;

Water washing: 33.5 g of distilled water was added to a water andsilylanization reagent vaporization tank which was then heated at atemperature of 160° C., water vapor was introduced into the reactiontube using N₂ for water washing, the linear velocity of N₂ in thereaction tube was 2 cm/s and the water washing time was 200 min;

Silylanization treatment: 2.4 g of hexamethyl disilylamine was added toa water and silylanization reagent vaporization tank which was thenheated at a temperature of 140° C., the hexamethyl disilylamine vaporwas introduced into the reaction tube using N₂ to react with the silicagel, the linear velocity of N₂ in the reaction tube was 0.5 cm/s, thesilylanization temperature was 250° C., and the silylanization time was120 min; the obtained catalyst was referred to as TS-B4.

The TS-B4 was evaluated. The catalyst was used for propylene epoxidationto produce propylene oxide, operated continuously for 750 hr, thereaction temperature was raised from the initial 50° C. to 80° C., andsampled for gas chromatography analysis. The EBHP conversion ratewas >99.9%, the highest selectivity to PO reached 97.5%, the averageselectivity reached 95.7%. The product was collected for ICP-OESanalysis, the content of the catalyst component Ti in the product wasabout 313 ppm; the Ti content in the fresh catalyst was about 3.42%, theTi content in the catalyst after evaluation was about 2.61%, and theloss rate reached 23.7%.

Comparative Example 2

30 g of silica gel carrier was weighed and charged into the reactiontube and dried at 240° C. for 240 min in N₂ atmosphere, and the linearvelocity of N₂ in the reaction tube was 2 cm/s.

Chemical vapor deposition of TiCl₄: 4.17 g of TiCl₄ was added to a TiCl₄vaporization tank which was then heated at a temperature of 145° C., theTiCl₄ vapor was introduced into the reaction tube using N₂ to react withthe silica gel, the linear velocity of N₂ in the reaction tube was 0.8cm/s, the deposition time was 200 min (the reaction temperature of thisstep was the same as the drying temperature in the previous step, whichwas 240° C.);

Calcination: the temperature was raised to 600° C. at a heating rate of2° C./min, the linear velocity of N₂ in the reaction tube was 2 cm/s,the calcination lasted for 200 min;

Water washing: 33.5 g of distilled water was added to a water andsilylanization reagent vaporization tank which was then heated at atemperature of 160° C., the water vapor was introduced into the reactiontube using N₂ for water washing, the linear velocity of N₂ in thereaction tube was 2 cm/s and the water washing time was 200 min;

Chemical vapor deposition of alkyl silicate: 27 g of tetraethylorthosilicate was added to a alkyl silicate vaporization tank which wasthen heated at a temperature of 180° C., tetraethyl orthosilicate vaporwas introduced into the reaction tube using N₂ to react with the silicagel, the linear velocity of N₂ in the reaction tube was 0.9 cm/s, andthe deposition time was 180 min (the reaction temperature of this stepwas the same as the temperature at which the alkyl silicate vaporizationtank was heated, which was 180° C.);

Calcination: the temperature was raised to 600° C. at a heating rate of3° C./min, the linear velocity of air in the reaction tube was 0.8 cm/s,the calcination lasted for 60 min;

Silylanization treatment: 2.4 g of hexamethyl disilylamine was added toa water and silylanization reagent vaporization tank which was thenheated at a temperature of 140° C., hexamethyl disilylamine vapor wasintroduced into the reaction tube using N₂ to react with the silica gel,the linear velocity of N₂ in the reaction tube was 0.5 cm/s, thesilylanization temperature was 250° C., and the silylanization time was120 min; the obtained catalyst was referred to as TS-01.

The TS-01 was evaluated. The catalyst was used for propylene epoxidationto produce propylene oxide, operated continuously for 1500 hr, thereaction temperature was raised from the initial 50° C. to 100° C., andsampled for gas chromatography analysis. The EBHP conversion ratewas >99.9%, the highest selectivity to PO reached 94.6%, the averageselectivity reached 92.1%. The product was collected for ICP-OESanalysis and no catalyst component Ti was found.

Comparative Example 3

30 g of silica gel carrier was weighed and charged into the reactiontube and dried at 240° C. for 240 min in N₂ atmosphere, and the linearvelocity of N₂ in the reaction tube was 2 cm/s.

Chemical vapor deposition of TiCl₄: 4.17 g of TiCl₄ was added to a TiCl₄vaporization tank which was then heated to a temperature of 145° C.,TiCl₄ vapor was introduced into the reaction tube using N₂ to react withthe silica gel, the linear velocity of N₂ in the reaction tube was 0.8cm/s, the deposition time was 200 min (the reaction temperature was thesame as the drying temperature in the previous step, which was 200° C.);

Calcination: the temperature was raised to 600° C. at a heating rate of2° C./min, the linear velocity of N₂ in the reaction tube was 2 cm/s,and the calcination lasted for 200 min;

Water washing: 33.5 g of distilled water was added to a water andsilylanization reagent vaporization tank which was then heated at atemperature of 160° C., the water vapor was introduced into the reactiontube using N₂ for water washing, the linear velocity of N₂ in thereaction tube was 2 cm/s and the water washing time was 200 min;

Calcination: the temperature was raised to 600° C. at a heating rate of3° C./min, the linear velocity of air in the reaction tube was 0.8 cm/s,the calcination lasted for 60 min;

Silylanization treatment: 2.4 g of hexamethyl disilylamine was added toa water and silylanization reagent vaporization tank which was thenheated at a temperature of 140° C., hexamethyl disilylamine vapor wasintroduced into the reaction tube using N₂ to react with the silica gel,the linear velocity of N₂ in the reaction tube was 0.5 cm/s, thesilylanization temperature was 250° C., and the silylanization time was120 min; the obtained catalyst was referred to as TS-02.

The TS-02 was evaluated. The catalyst was used for propylene epoxidationto produce propylene oxide, operated continuously for 620 hr, thereaction temperature was raised from the initial 50° C. to 95° C., andsampled for gas chromatography analysis. The EBHP conversion ratewas >99.9%, the highest selectivity to PO reached 93.6%, the averageselectivity reached 91.1%; the Ti content in the fresh catalyst wasabout 3.45%, the product was collected for ICP-OES analysis, the Ticontent in the catalyst after evaluation was about 2.56%, and the lossrate reached 25.8%.

The experimental results of the examples and the comparative examplesshow that the catalysts prepared by the preparation methods of thepresent disclosure had good catalyst stabilities during use, theactivities and selectivities of the catalysts did not changesignificantly during the observation time, and the activities werestable; Ti element was not found in the product of each example,indicating that the active component in the catalyst was not lost. Itcan be seen that the catalysts prepared by the preparation methods ofthe present disclosure can reduce the loss of the Ti active centersduring use, the catalyst activities were stable, the service lives ofthe catalysts were improved; and the catalysts had high selectivities toPO.

1. A preparation method for an olefin epoxidation catalyst, whichcomprises the following steps: (1) loading an auxiliary metal salt ontoa silica gel carrier to obtain an auxiliary metal salt modified silicagel carrier A; preferably, using a impregnation method to load theauxiliary metal salt onto the silica gel carrier; (2) carrying out adrying treatment for the A obtained in step (1); (3) carrying out achemical vapor deposition for the dried A using a titanium salt vapor,preferably TiCl₄ vapor to obtain a silica gel B on which the auxiliarymetal salt and the titanium salt, preferably TiCl₄ were loaded; (4)calcining the B obtained in step (3) to obtain a silica gel C on whichthe auxiliary metal salt and Ti species are loaded; (5) carrying out awater vapor washing for the C obtained in step (4) to obtain aTi-MeO—SiO₂ composite oxide; (6) carrying out a vapor deposition for theTi-MeO—SiO₂ composite oxide using an alkyl silicate vapor to obtain theTi-MeO—SiO₂ composite oxide D having a silicon-containing compoundloaded on the surface of the composite oxide; preferably, the alkylsilicate vapor is tetraethyl orthosilicate vapor; (7) calcining the Dobtained in step (6) to obtain a Ti-MeO—SiO₂ composite oxide having aSiO₂ layer coated on the surface of the composite oxide, which isreferred to as SiO₂—Ti-MeO—SiO₂; (8) carrying out a silylanizationtreatment for the SiO₂—Ti-MeO—SiO₂ obtained in step (7).
 2. The methodaccording to claim 1, wherein the auxiliary metal salt in the step (1)is selected from the group consisting of Ce(NO₃)₃, Pr(NO₃)₃, Tb(NO₃)₃,La(NO₃)₃ and combinations thereof; preferably, in step (1), using theamount of the metal oxide of the auxiliary metal salt for calculation,the auxiliary metal salt is added in an amount ranging from 0.6-2.4 wt %based on the mass of the silica gel carrier.
 3. The method according toclaim 1, wherein the silica gel carrier used in step (1) is a C-typesilica gel, preferably a spherical shape or a block C-type silica gel,more preferably an irregular blocky C-type silica gel; preferably, thesilica gel carrier used in step (1) has a specific surface area of100-350 m²/g, an average pore diameter of 8-11 nm, a pore volume of0.7-1.2 ml/g, a Na₂O impurity content of <100 ppm, a Fe₂O₃ impuritycontent of <500 ppm and a size of a spherical equivalent diameter of0.5-2 mm.
 4. The method according to claim 1, wherein in step (2), thedrying temperature is 150-240° C. and drying time is 120 min-240 min;preferably, in step (2), using a drying gas to carry out the drying, andthe drying gas is a gas that does not react with silica gel, preferablyair or nitrogen; more preferably, the drying in step (2) is carried outspecifically in a reaction tube, and the flow rate of the drying gas inthe reaction tube is preferably 1.0-3.0 cm/s.
 5. The method according toclaim 1, wherein based on the weight of the silica gel carrier used instep (1), in step (3), Ti is loaded on the silica gel carrier in anamount ranges from 0.1-5.0 wt %, preferably 2.5-4.5 wt %; preferably,the titanium salt vapor used in step (3) has a temperature of 137°C.-150° C.; preferably, the chemical vapor deposition of step (3) iscarried out in a reaction tube, the dried A is charged in the reactiontube, an inert gas, preferably N₂ is used to introduce the titanium saltvapor into the reaction tube, the inert gas has a flow rate of 0.05-2.0cm/s, preferably 0.50-1.35 cm/s in the reaction tube, the temperature ofthe reaction is 150-300° C., and the deposition time is 120-240 min. 6.The method according to of claim 1, wherein the calcination in step (4)is carried out in N₂ atmosphere, at a calcination temperature rangesfrom 450-700° C., preferably 500-600° C., a calcination time ranges from30-240 min, preferably 120-180 min, the flow rate of N₂ is 0.05-2.0cm/s, preferably 1-2.5 cm/s.
 7. The method according to of claim 1,wherein the water vapor used for water washing in step (5) has atemperature of 100-200° C., preferably 120-180° C.; based on the amountof the Ti element in the titanium salt vapor used in step (3), the molarratio of the water vapor to Ti is 20-150:1, preferably 50-100:1, thewater washing time is 180-240 min; the water vapor washing of step (5)is preferably carried out in a reaction tube, water vapor is introducedinto the reaction tube using an inert gas, preferably N₂, the inert gashas a flow rate of 1-2.5 cm/s.
 8. The method according to claim 1,wherein the alkyl silicate vapor used in step (6) is heated to atemperature of 166-200° C.; the vapor deposition of step (6) is carriedout in a reaction tube, and the alkyl silicate vapor is introduced intothe reaction tube using an inert gas, the inert gas is preferably N₂;the flow rate of the inert gas in the reaction tube is 0.05-2.0 cm/s,preferably 0.5-1.0 cm/s, the reaction temperature is 166-200° C., thedeposition time is 120-180 min, and the weight ratio of alkyl silicateto the silica gel carrier used in step (1) is 0.5-1:1.
 9. The methodaccording to claim 1, wherein the calcination in step (7) is carried outin air atmosphere at a temperature of 500-700° C., with a calcinationtime of 30-120 min, and the flow rate of air is 0.5-1 cm/s.
 10. Themethod according to claim 1, wherein the silylanization reagent used forthe silylanization treatment in step (8) is hexamethyl disilylamine,based on the weight of the silica gel carrier used in step (1),hexamethyl disilylamine is used in an amount of 5 wt %-15 wt %;preferably, the temperature of hexamethyl disilylamine used in step (8)is 126-150° C.; preferably, the silylanization treatment is carried outin a reaction tube, the silylanization reagent vapor is introduced intothe reaction tube using an inert gas, preferably N₂, the flow rate ofthe inert gas in the reaction tube is 0.5-1 cm/s, the silylanizationtemperature is 200-300° C., and the silylanization time is 60-180 min.11. A method for catalyzing propylene epoxidation to prepare propyleneoxide, comprising using the catalyst prepared by the method according toclaim
 1. 12. The method according to claim 11, wherein the processconditions for catalyzing propylene epoxidation to prepare propyleneoxide are as follows: a reaction temperature of 40-120° C., a gaugepressure of 2-4.5 MPa, a molar ratio of propylene to ethylbenzenehydroperoxide of 3-10:1 and a mass space velocity of 1-5 h⁻¹.